OX40 (TNFRSF4), also known as ACT35, IMD16, TXGP1L, and CD134, is a 50-kD type I transmembrane glycoprotein in the TNFSFR family of costimulatory receptors expressed on activated CD4+ T cells. In the context of cancer, OX40-expressing activated T cells are found in tumor infiltrating lymphocytes. OX40 and its ligand, OX40-L, play a crucial role in inducing and maintaining T-cell responses. Recent studies have demonstrated the utility of enhancing anti-tumor T cell function to fight cancer, with key components of an effective response including the activation of CD4+ T cells and promoting survival signals through memory and effector T cells. Given the ongoing need for improved strategies for treating diseases such as cancer through, e.g., enhancing immune responses such as T cell responses, novel agents that modulate T cell responses, such as those that target OX40, as well as therapies (e.g., combination therapies) that use such agents, would be therapeutically beneficial.